Asymmetric Alkylation of Aldehydes Catalyzed by Novel Dinuclear Bis-BINOLate Titanium(IV) Complexes

Bis-BINOLs 1a , b in which two BINOL units are tethered by o - and m -phenylenebis(ethynyl) groups form stable dinuclear bis-BINOLate titanium(IV) complexes 2a , b by treatment with titanium tetraisopropoxide. In the presence of excess titanium tetraisopropoxide, 2a and 2b (2–20 mol %) catalyze diethylzinc addition to aromatic and aliphatic aldehydes in an efficient manner to give the ethylation products with high enantioselectivities. While more than 1 equiv of titanium tetraisopropoxide (with respect to a substrate aldehyde) is generally employed for obtaining high turnover frequency and selectivity in reactions catalyzed by a parent (BINOLate)Ti(O i Pr) 2 , the amount can be reduced as low as 0.2 equiv in the reactions catalyzed by 2a , b .